This invention provides a gradually effective, coated fertilizer and is used in the field of the fertilizer industry.
Generally, chemical fertilizers are immediately effective and it is difficult to maintain the effect of fertilization for a long time. Much fertilization as once made has such disadvantage as high salt concentration disturbance upon crops and water pollution due to leaking of fertilizer. To overcome the disadvantages, gradually effective fertilizers coates with various resins have been so far proposed.
In order to prepare gradually effective, coated fertilizers by spray-coating, the following three coating resin systems are usually suitable for the coating material.
(1) A resin solution dissolved in organic solvent, PA1 (2) An aqueous solution of a water-soluble resin, and PA1 (3) An aqueous emulsion of resin
In case of preparation of a coated fertilizer with the above-mentioned coating resin systems, various problems arise.
That is, in case of (1), the raw material cost is higher, and there are fears of air pollution and fires during the preparation of fertilizers owing to the use of an organic solvent. In recovering the solvent, facility therefor is further required, total facility cost increasing considerably.
In case of (2), the resin is water-soluble, and thus the resin to be used as the protective film has a limit in the function of lowering the velocity of releasing the effective components of a fertilizer into the soil.
In case of (3), there are no problems as encountered in cases of (1) and (2), but the coating material of aqueous emulsion system has the following problems.
.circle.1 Formation of continuous film:
In the aqueous emulsion of a resin, fine resin particles are dispersed in water. In order to render the effective components of a fertilizer gradually releasable, it is necessary to coat the fertilizer with a uniform film. To this end, each of the emulsion particles must coalesce to form a satisfactorily continuous film.
.circle.2 Problem of caking of coated fertilizer grains:
On condition that the coating film has a low glass transition temperature, when the coated fertilizer is exposed to an elevated temperature or a load, for example, when sacks of fertilizer are piled up in the summer, the coated fertilizer grains undergo fusion on the coating film surfaces, resulting in caking.
In order to prevent the caking of the coated fertilizer grains themselves, it is preferable to select a resin system having a high softening temperature, but generally speaking, such a resin system has difficulty in forming the continuous film of .circle.1 , and the gradual releasability becomes worse. On the other hand, when the formation of a continuous film is to be facilitated and the gradual releasability is to be improved by means such as lowering the glass transition temperature of resin and adding a film-forming aid or a plasticizer thereto, the coating film is liable to undergo softening and fushion, and coated fertilizer grains are liable to become caking.
Thus, it has been difficult to form a uniformly continuous film and to maintain distinguished gradual-releasability, while preventing caking of the coated vfertilizer grains.
As a result of extensive studies to solve these problems, it has been found that said problems can be solved by using a coating material comprising an aqueous emulsion of a resin as the main component and having a glass transition temperature of 60.degree. C. or higher, or by using a coating material which is prepared by using an aqueous emulsion of a thermoplastic resion together with a melamine resin and subjecting the coated fertilizer to heat treatment at 70.degree. C. or higher. The present invention has been based on these findings.